Services between London and Stevenage have been disrupted by damage to overhead wires in the New Barnet area.
This has caused some lines to be blocked, and services may be cancelled, revised or delayed.
Disruption is expected until the end of the day.
Operators affected include Grand Central, London North Eastern Railway, Great Northern, Hull Trains, Lumo, and Thameslink.
Each operator has provided specific advice which can be checked on their websites or via National Rail.
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